According to The East Bay Express:
"A fan group called “We Stand With Oakland” said it has been contacted by unnamed law firms to file legal action against the team and the NFL, with the hope to either keep the Raiders, get an expansion franchise or, if the team leaves, demand the league pays off the debt at the Coliseum."
The bottom line is that they feel that the Raiders should pony up for the $83 million renovations to Mt. Davis if they are are going to be leaving for Vegas. A similar lawsuit was filed in St. Louis against the Rams and is moving on despite the attempts to block it by the team and the NFL. Lawyers feel that the same can happen for Oakland. They want to either stop the team from moving (unlikely), get the cash back, or acquire a new franchise for the city.